It is widely known that administration of nitric oxide (NO), or compounds which deliver NO (i.e., NO donors, NO producing agents) to a subject, can provoke powerful vasodilator responses. Such administration is often accompanied by a number of undesirable side effects which include headache, flushing, and hypotension.
The physiological role of NO has been described as that of a powerful chronic vasodilator agent based on there being a marked increase in vascular tone following NO synthase (NOS) blockade (Johnson et al., Am. J. Hypertens. 5:919, 1992; Tolins et al., Hypertens. 17:909, 1991). The role of NO as a chronic vasodilator has only been inferred by indirect means, i.e., by removal of NOS activity. Endogenously, much more multiplicity and overlap in the control of vasodilation can be inferred from the scientific literature. For example, vasodilation can be induced by acetylcholine, bradykinin, adenosine, adenosine triphosphate (ATP), histamine, vasoactive intestinal polypeptide (VIP), and leukotrienes, amongst others. The actions of these endogenous modulators have been shown to be dependent, at least in part, on the presence of the endothelium, an effect likely mediated by endothelial derived relaxing factor/NO (EDRF/NO) (Garg, U. C. et al., J. Biol. Chem. 266:9, 1991; Garg, U. C. et al., J. Clin. Invest. 83:1774, 1989; Palmer, R. M. J. et al., Nature 327:524, 1987). Other vasodilator mechanisms exist which are not endothelium dependent, such as β2-adrenergic receptor activation, atrial natriuretic peptide (ANP) and certain prostaglandins. The actions of NO have been suggested to be mostly cGMP-mediated via guanylate cyclase activation, although other mechanisms have been suggested. For example, Garg et al. (J. Biol. Chem. 266:9, 1991; J. Clin. Invest. 83:1774, 1989) and others (Assender, J. W. et al., J. Cardiovasc. Pharmacol. 17(Suppl. 3):S104, 1991; O'Conner, K. J. et al., J. Cardiovasc. Pharmacol. 17(Suppl. 3):S100-103, 1991) demonstrated a difference in the effects of NO-generating vasodilator agents in inhibiting vascular smooth muscle cell growth in culture; however, it is clear that NO can act not only as a vasodilator but also to inhibit vascular growth responses in a number of conditions (Farhy, R. D. et al., Circ. Res. 72:1202, 1993).
It has been believed and widely practised that NO, in humans and animals, produced via sodium nitroprusside (SNP) infusion, causes vasodilation in peripheral vasculature at doses greater than 10 μg/kg per min. It has recently been determined that NO also performs a function through interaction with endothelin (ET) (Banting et al., J. Hypertens. 14:975, 1996; Richard et al., Circulation 91:771, 1995). Prior to this time, ET had been believed to play a minimal role in maintaining tone in the peripheral microvasculature and to have little impact on the state of contraction of smooth muscle in those vessels. Recent studies have indicated (Banting et al., J. Hypertens. 14:975, 1996) that ET is under the inhibitory control of NO and that administration of NOS inhibitors results in elevated levels of ET.
Endothelins were first described in 1988 and have been shown to be powerful vasoconstrictors, predominantly found in vascular endothelium and, since that time, numerous ET antagonists and their pharmaceutically acceptable salts have been identified and can be obtained commercially (e.g., Sigma, American Peptides). Detailed descriptions of the chemical structures of various ET antagonists may be found in U.S. Pat. No. 5,284,828 issued 8 Feb. 1994 to Hemmi et al., U.S. Pat. No. 5,378,715 issued 3 Jan. 1995 to Stein et al., and U.S. Pat. No. 5,382,569 issued 17 Jan. 1995 to Cody et al. In addition, U.S. Pat. No. 5,338,726 issued 16 Aug. 1994 to Shinosaki et al. describes the chemical structure of ET converting enzyme inhibitors. To date, however, antagonists of ET have not been approved for therapeutic use, although a number of investigators have postulated that ET antagonists could be used for conditions ranging from renal failure, endotoxic shock, asthma, angina, or diabetes to pulmonary hypertension and possibly other indications.
Under normal physiological conditions, ET can be found in almost all parts of circulation at very low levels. In general, in the normal rodent circulation ET is not found in elevated quantities and appears to have little detectable role in the normal regulation of vascular tone, i.e., there is no appreciable decrease in blood pressure when an ET antagonist is administered by injection in normal circulation. Further, at present there does not appear to be any evidence suggesting that ET plays a physiological role even in a small portion of the circulation under normal circulatory conditions in experimental models. However, it is likely that the systemic circulation may appear to be normal when, in fact, specific regions of the circulation are undergoing pathophysiological changes such as occurs in conditions such as erectile dysfunction (ED) (Adams et al., Int. J. Impot. Res. 9:85-91, 1997).
Consequently, there are cardiovascular conditions which are traditionally treated in human beings by significant doses of NO or NO donors, such as glyceryl trinitrate (GTN) (0.2 mg/h and greater). However, these doses are known to induce systemic vasodilation and provoke considerable overall systemic side effects (The, L. S. et al., Brit. J. Rheum. 34:636, 1995). This is particularly so where a pathological condition exists only in certain major organs (e.g., heart, kidney, liver). As a result, a satisfactory method for promoting recovery of normal perfusion pressure in organs with certain pathologies without producing overall systemic hypotension has not been discovered.
Based on the understanding that a significant portion of underlying problems in clinical erectile dysfunction relates to “vascular” mechanisms, much of the current state-of-the-art research involves determining the contribution that the different vascular effector control systems make in normal and pathophysiological states. There is substantial understanding of hemodynamic events that lead to an erection, and yet the quantitative roles of each of the neuroeffector, humoral and local systems in these events remain poorly described. Since 1990, NO has been considered the primary non-adrenergic non-cholinergic neurotransmitter in the penis and has been presumed to be the primary mediator of corporal relaxation during erection (Ignarro L. J. et al., Biochem. Biophys. Res. Comm., 170:843, 1990).
It is well established that, for an erection to occur, neurally mediated (autonomic) vasodilation of the penile arterial blood vessel and the trabecular meshwork takes place (Lue, T. F. et al., J. Urol. 137(5):829, 1987) permitting increased blood flow into the cavernous bodies of the penis. The expanding intra corporal volume compresses the effluent veins that lie between the erectile tissue and the surrounding fibrous, relatively inelastic, tunica albuginea. The outflow capacity is thereby decreased and entrapment of blood ensues, resulting in the transformation of the flaccid penis into its erect state (Lue, T. F. et al., J. Urol. 137(5):829, 1987; Juenemann, K. P. et al, J. Urol. 136(1):158, 1986; Lue, T. F. et al., J. Urol. 130:1237, 1983; Weiss, H. et al., Ann Intern. Med. 76:793, 1980). The level of arterial vascular tone (i.e., blood pressure) is one of critical importance in this process, although adequate perfusion pressure is also a necessary factor. The converse, detumescence, is mediated by the sympathetic nervous system (Saenz de Tajada, I. et al., Am. J. Physiol. 254:H459, 1988; Juenemann, K. P. et al., Br. J. Urol. 64, 1989).
The issue of “impotence” (defined as “a pattern of persistent or recurrent inability to develop or maintain an erection of sufficient rigidity for successful coitus”) was discussed at a consensus conference of the National Institutes of Health (NIH) in Washington in December 1992 and has been clearly identified as having a wide range of causative or associated factors. The Massachusetts Male Aging Study (MMAS) has provided an updated view of the epidemiology of erectile dysfunction. It is accepted that the prevalence of impotence increases with age (Kinsey A. C. et al., “Sexual Behaviour in the Human Male”, W.B. Saunders: Philadelphia, 1948). Severe or complete ED increases from 5 to 15% between 40 and 70 years of age, (Feldman, H. A. et al., J. Urol. 151:54, 1994). ED has been shown to be “directly correlated with heart disease, hypertension, diabetes, associated medications, indices of anger and depression, and inversely with serum dehydroepiandrosterone, high density lipoprotein, cholesterol and an index of dominant personality.”
It is now estimated that in North America there are more than 30,000,000 men with some form of ED, a significant increase from the figure of 10,000,000 quoted just 10 years ago (Shabsigh, R. et al., Urology 32:83, 1988; Whitehead E., Geriatrics 43(2):114, 1988; Furlow, W. L. et al., Med. Aspects Human Sexuality 19:13, 1985). From these figures it is also reasonable to estimate that as many as three million Canadian men may have a degree of ED. The direct cost of treating impotence is significant. Reliable figures from 1985 show that the cost of treating impotence exceeded 146 million dollars in the United States in that year alone (National Center for Health Statistics) and this number is just the estimated market size for one type of injectable therapy. The secondary effects and indirect costs associated with ED suggest that impotence and sexual dysfunction are medical icebergs. The consequences of sexual dysfunction may be seen in strains on the host relationship potentially leading to marital breakdown, violence, work related sequelae, deviant sexual behaviour and impacts on children, when present, that can carry the damage into a new generation of unwanted behaviours. If ED underlies even a small but significant percentage of marital and family breakdown, then it adds vastly to the social and economic burden in society. The pragmatic issue is that large numbers of men are now being treated for ED and most of the treatments are fairly blunt instruments (e.g., intracavernosal injection (ICI)) of mixed vasoactive compounds, penile prosthesis insertion) with significant cost and complications (ICI: pain, priapism, dislike of the technique; prostheses: reoperation, infection, distortion of body image).
Administration of NO, or compounds which are able to deliver NO, have been suggested as possible therapies; however, these agents can provoke powerful yet inappropriate vasodilator responses (Brock et al., J. Urol. 150:864, 1993). Such administration is often accompanied by a number of undesirable side effects related to systemic vasodilation which include headache, flushing, and hypotension. Consequently, there is a real need to provide methods whereby ED and other vascular disease may be quickly and effectively treated without any inappropriate side effects.